It is now possible to introduce foreign genes of interest into animal chromosomes in vitro (e.g. in the test tube) using various known vectors (see e.g. U.S. Pat. Nos. 4,650,764 and 4,736,866; J. Wilson et al., 85 P.N.A.S. U.S.A. 3014-3018 and 4421-4425 (1988); J. Dougherty et al., 84 P.N.A.S. U.S.A. 1197-1201 (1987)). In some in vitro approaches, genes are placed in somatic cells (e.g. cells from the liver), while in other approaches, genes are placed in fertilized germ cells to create transgenic animals. The disclosures of these articles and patents, and of all other articles recited herein, are incorporated by reference as fully set forth herein. However, such in vitro prior art techniques can be time consuming and require great laboratory skill to use, can (in the case of transgenic techniques) result in all of the germ and somatic cells throughout the body expressing the foreign gene (with resulting potential side effects and safety concerns), and can lead to difficulties in obtaining government approvals (since progeny will also carry the modified gene in their chromosome).
While there therefore have been efforts to transform selected animal cells in vivo (using live animals), difficulties have arisen in achieving this result. One stumbling block related to the need for a high titer of virus vector (e.g. retrovirus vector) where virus vectors were used. Accordingly, there were efforts to increase the titers of vectors. See e,g. J. Price et al., 84 P.N.A.S. U.S.A. 154-160 (1987).
While high titer retrovirus vectors have recently been used (albeit inefficiently) in vivo to infect certain neonatal retinal cells adjacent the brain (see D. Turner et al., 328 Nature 131-136 (1987)), other cells of live animals, and cells of live animals that are past the neonatal stage, have proved resistant to in vivo recombinant techniques.
Thus, it can be seen that a need exists for the development of efficient means for rendering other cells (particularly mammary epithelial cells) recombinant in vivo.